Ultra short wave device



May 14, 1940. E. D. McARTHUR ULTRA SHORT WAVE DEVICE Filed Dec. 31, 1938 Inventor Elmer D. Mc Arthur,

His ttorney.

Patented May 14, 1940 PATENT OFFICE ULTRA SHORT WAVE DEVICE Elmer D. Schenectady, N. Y., assignor to General Electric Company, a corporation of New .York

Application December 31, 1938, Serial No. 248,771

, 6 Claims;

The present invention relates to improvements in high frequency apparatus.

It is an object of the invention to provide a monitoring system which is'usable at. ultra-high frequencies for maintaining the output level of a high frequency amplifier substantially constant. In its preferred embodiment the invention is used in combination with a tube having means for producing a concentrated electron beam and for modulating the beam at ultra-high frequencies. An important feature of the invention consists in the provision of novel means for regulating the modulation of the beam in accordance with conditions prevailing at the output terminals of the tube.

The features which'I desire to protect herein are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the drawing, in which the single figure represents a sectional view of a discharge device to be regulated and a monitoring system for accomplishing such regulation.

Inasmuch as the invention is considered to be primarily applicable to discharge devices of the character describedand claimed in W. C. Hahn application Serial No. 153,602, filed July 14, 1937, it will be helpful to refer briefly to some of the principles utilized in such devices.

An electron stream, such as flows between .the electrodes of a vacuum tube, may be modulated either as to electron velocity or as to charge density. The first type of modulation involves the production of systematic irregularities in electron velocity from point to point along the beam. The second involves the production of charge density variations, such variations being manifested as'systematic irregularities in the electron grouping.

In the conventional design of electronic discharge devices no-dlstinction is made between.

, these two types of modulation. In connection with ultra-short-wave devices, however, it is advantageous to utilize modulating electrodes which are capable' of producing velocity modulation without simultaneously causing appreciable charge density variations. For reasons which '50 need not be elaborated here this expedient. avoids the'large input losses which are observed with conventional prior art devices when they are operated at extremely high frequencies. By additional means, also described in the aforesaid as Hahn application, velocity modulation produced as above specified may be subsequently converted into charge density modulation of a higher order of magnitude so as to produce amplification effects.

It is found that the velocity modulation prin- 5 ciple may be most readily utilized in a discharge device of the cathode ray type, wherein theelongated stream of electrons is susceptible of being variously influenced at different points along its length. I have, therefore, chosen a device of this 10 kind to illustrate my present invention, in the single figure of the drawing Referring particularly to the drawing I have shown an electron beam tube which comprises an evacuated envelope having an elongated shaft 15 portion l0 and an enlarged anode-containing portion N. This envelope may be suitably constituted of glass, quartz, or any equivalent insulating material.

The shaft portion In encloses means, such as a 20 known type of electron gun, for producing an electron beam. The combination shown comprises a cathode M, which is indicated in dotted outline and a focusing cylinder i5 for confining the electrons from the cathode to a concentrated beam. This cylinder may be either connected directly to the cathode or maintained at afew volts negative with respect to it. In order to accelerate the electrons to a desired extent, there is provided an accelerating electrode I6 which is 30 spaced from the cathode and which may be biased to a suitable positive potential, say several hundred volts.

In, order that the intermediate portion of the beam path may be maintained at a desired potention level there are provided a number of intermediate electrodes 2I which suitably comprise rings of conducting material applied to the inner wall surface of the envelope. These'are provided with external contact-making terminals 23. A 40 number of magnetic focusing coils 25 distributed along the envelope serve to prevent dispersion of the electrons and to maintain .thebeam in focus during its passage through the discharge space. In some cases thesecoils may be advantageously replaced by electrostatic beam focusing means. 7

After traversing the envelope, the electron beam is collected by an anode l8 which consists of graphite or other suitable material. A tubular electrode I9 in the nature of a suppressor grid serves to prevent secondary electrons emitted by the anode from returning to the discharge space. In the operation of the device the intermediate electrodes 2| may be maintained at ground potential, the cathode II at one thousand to several thousand volts below ground, and the anode l8 at one thousand to several thousand volts positive with respect to the cathode. The suppressor. grid I9 should be biased fifty to several hundred volts negative with respect to the anode l8. These potential relationships may be'established by means of a suitable source-of potential, conventionally represented as a battery 21 connected as shown.

The combination of elements so far described comprises means for producing a unidirectional electron beam of substantially constant averageintensity and velocity; As pointed out in the aforesaid Hahn application, Serial No. 153,602, an electron beam of this type may be velocity modulated by applying to the beam longitudinal ,potential gradients which vary cyclically at a desired frequency. One suitable velocity modulating structure is shown in the drawing.

This comprises a modulating chamber or space provided between thev extremities of two conduct ing tubular members 3i and 32 which are arranged to surround the beam path. Within this space there is provided a tubular control electrode 30 which also surrounds the beam path. Thetubular/members 3,] and 32 are shown as being grounded so that the boundaries of the modulating space may be regarded as definitely fixed. By alternately raising and lowering the potential of the electrode 30 with respect to these boundaries, variable potential gradients are producedwhich act longitudinallyon the electron beam as it traverses the approach spaces between the electrode 30 and the extremitiesof the members 3| and 32. The modulating efiect thus produced will be most pronounced if the length of the tubular electrode 30 is socorrelated to the velocity of the beam that the electron transit time therethrough corresponds at least approximately to a half cycle of the controlpotential (or to an odd number of such half cycles). If this condition is fulfilled, an electron which enters the modulating space when the potential of the .con-

' the electrode and again as it leaves the electrode trol electrode 30 is a maximum is accelerated first by the gradient existing between the tube 3| and 9. half cycle later when the electrode potential is at av minimum with respect to the tube. 32. Similarly, an electron which enters the modulating I space in such time phase as to be retarded by the.

effect of the control electrode is also retarded as it leaves the electrode. As a result of these efi'ects, the electron beam leaving the modulating chamber is made up of alternate elements, some of which have a velocity above the average of the beam and others a velocity below such average.

Modulating potential may be applied to the control electrode 30 from any desired source (not shown) and for the use herein contemplated will ordinarily include a carrier which has been previously modulatedwith an intelligence-conveymg signal, such as an audio or video signal. As ,a means for connecting this potential to the control electrode structure there is provided a concentric conductor transmission line comprising an inner conductor 35 and an outer conductor 36 which concentrically surrounds the inner conductor;

If only weak control potentials-are available, the velocity modulation produced may be relatively slight. However, it may be converted into charge density'modulation of a higher order of magnitude by a mechanism now to be described.

It will be understood that-as the beam issues from the modulating space it comprises alternate groups of slow and fast electrons. At the exit boundary of the modulating chamber the beam is still substantially uniform so far as charge density or electron grouping is concerned. At a 5 slightly 'latertime, however, the more rapidly moving electronswill catch up with' the slower electrons, and electron bunches will exist from point to point along the beam. The resultant succession of charge density maxima and minima' 10 closed within the conducting tube 32.

If this tube, which is shown broken away in 25 order to economize space on the drawing, is made sumciently long, a relatively slight degree of velocity modulation may be converted into a much higher order of charge density/modulation so that an amplification effect is obtained. However, if 30 the initial control voltage or signal is very small, a single stage of such amplification may still. yield f an insufilcient output. It is, therefore, advantageous to provide additional. means by which multistage amplification can be accomplished. 35 In the embodiment shown, this means comprises a second or modulation intensifying" chamber formed between the opposed extremities of the tube 32' and of another conducting tube 38. Within this chamber there is provided a tubular 40 electrode 40 generally-similar to the electrode 30 which has already been described.

It will be readilyunderstood that the current variations in the charge density modulated beam traversing the approach spaces which. exist be- 45 .tween theextremities of the electrode 40 andof the fixed potential tubes 32 and 38 will induce in the electrode a cyclically varying current of frequency corresponding to the modulation frequency'. The magnitude ofthis induced current will be greatest if the length of the electrode 40 corresponds approximately to the spacing between adjacent charge density maxima' and minima in the beam 'so,that the approach of a charge density maximum-corresponds with the recession of a charge density minimum and vice versa. In order that the induced current may be caused to produce the efiects desired in the prescut-connection, the "electrode vIII and the tubes 32 w and '38 (which are both at ground potential) should be connected through a high impedance circuit. In the arrangement illustrated, such a circuit is provided by the use'of two resonant transmis'sion'linesof the concentric conductor 5 type. i

The first of these comprises an inner conductor 42, which is connected with the electrode 40,

and an outer tubular conductor 43 which (3011-,

centrically surrounds the inner conductor. The 7 v inner and outer conductors are directly connected to one another at one end, as indicated at 44, so that the point of connection is approxi-' mately a quarter wave length from the opencircuited end of the transmlssion line; that is, I

the end to which the electrode 40 is connected.

The second transmission line is of the half wave type and comprises concentric conductors and 5|. These are connected at one end to the .electrode '40 and its associated parts and at the end are coupled to a discharge tube 60 whose nature and funtcion will'be more fully described hereinafter.

With an arrangement such as that indicated, the current induced in the electrode 40 will produce sustained oscillation of the transmission lines described above and will cause a voltage maximum or anti-node to exist between the electrode 40 and the adjacent extremities of the tubes 32 and 38. This voltage will be of cyclically varying character and will have a frequency determined by the rate of approach and recession of charge density maxima in the beam; that is to say, by the frequency of the initial velocity modulating potential.

By analogy with the operation of the electrode 30 it will be seen that the potential gradients produced by the electrode 40 will necessarily act to cause additional velocity modulation of the electron beam. Furthermore, since the voltage swing of the electrode 40 may be very much greater than that of the input electrode 3D,.the magnitude of the new velocity modulation is correspondingly larger than that of the initial modulation.

As a result of the amplifying operation described in the foregoing the beam issuing from the electrode 40 is highly velocity modulated according to a modulation pattern determined by that of the signal voltage initially impressed on the input electrode 30. In the drift space provided by the conductive tube 38 this velocity modulation may be converted into charge density modulation as previously explained. The tube 38 may be of any lengthrequired to accomplish the conversion effectively.

Output power is taken from the beam by means of a third tubular electrode 46 positioned between the extremity of the tube 38 and the opposed end of another conducting tube 41. As

I explained in connection with electrode 40, the

action of the modulated beam in traversing the electrode 46 is to induce in that electrode currents which correspond to the modulation of the beam.

In order that this effect may be a maximum, the

axial length of the electrode should be approximately equal to the spacing between adjacent charge density maxima and minima' in the beam.

The current variations induced in the electrode 46 may be effectively utilized by connecting the electrode to a high impedance circuit. This may be, for example, a resonant transmission line provided by a pair of concentric conductors 52 and 53. Assuming that these conductors are at I least approximately a quarter wavelength long,

they will comprise an oscillating circuit having a voltage maximum at the end nearest the electrode and a current maximum at the'opposite end. The power extracted from the beam by electrode 46 may be utilized by inductivelycoupling the remote end of the transmission line As a part of such means there is provided an arrangement for automatically producing a regulatory adjustment of the condition of resonance of the transmission line 50, 5|. It will be understood that any such adjustment, that is, any change in the tuning of the transmission line will produce a corresponding effect on the secondary modulation of the beam of the tube It which is caused by the electrode 40. Thus, if maximum modulation is obtained when the transmission line is perfectly resonant, any detuning of the line will tend to lessen the amplitude of modulation. Furthermore, such a change in modulation is reflected directly ,in a corresponding change in the output level at the electrode 46.

For varying the condition of resonance of the line 50-5I. in a convenient manner, there is provided another beam tube 60 which is adapted to provide an electron beam in proximity to the extremities of the conductors 5i and 5i. Efiective coupling of the conductors and the beam is accomplished by passing the beam axially through a series of tubular conductors 63, 64 and 65 which are respectively connected to the inner and outer conductors of the transmission line.

The tube 60 is in many respects similar to the device I0 previously described and includes a cathode 61, an accelerating electrode 68, an anode t9, and a suppressor grid Hi. There are also providing afocusing coil 13 and an intermediate electrode M which correspond in function with the elements 2! and 25 described above. A bat tery l6 serve's to maintain a desired D. C. potential relationship between the various electrodes referred to. M

It is apparent that voltage variations of the electrode 63 resulting from the oscillation of the transmission line 505l will produce velocity changes in the electron beam passing through the tube 60 in line with the principles already set forth. This modulating action represents an absorption of high frequency power by the electron beam and tends to modify the condition of resonance of the transmission line in accordance with the power absorbed. Since this latter quantity is itself a function of the current in the .beam, changes in the beam current necessarily change the condition of resonance of the line. to say, if the transmission line is in perfect resoname for one value of the beam passing through the tube 60, it will necessarily be somewhat out of resonance for any other value of such current.

Correlatively, the action of the transmission electron stream flowing through the tube III will also be a function of the beam current in the tube 60. My invention takes advantage of this fact by providing means for varying'the current in the tube 60 in accordance with a monitoring voltage derived from the output circuit of the tube III. The arrangement by which this is done will now be described.

The beam current in the tube 60 is directly controlled by a tubular electrode BQwhich-is positioned to surround the cathode 61 and which serves the function of a control grid. It is normally biased to a positive potential by means of a battery 8 I. Its instantaneous potential level with respect to the cathode 61 is regulated, however, by 'means'of a circuit coupled to the output of the tube l0. I

The circuit referred to above comprises a nonresonant type transmission line 83, 84 which is inductively coupled-t0 the electrode 46. The volt- That is age thereby developed is impressed across a rectifying circuit which includes a two-electrode rectifier II and a resistor-condenser combination 86,

I! connected in series with the rectifier. With this arrangement the voltage appearing across the terminals of the resistor-condenser combination is determined by the average output level at the electrode 46. The time constant of the combination should be high enough so that the volt? age across it does not follow the cyclical variations of the intelligence-conveying or audio frequency with which the signal is modulated, It should be low enough, however, so that the voltage across the combination can respond promptly to definite changes in the average signal level as determined by variations in the carrier voltage.

By means of a pair of circuit conductors 88 the rectified voltage produced by the circuit combination described in the foregoing is impressed across a condenser 89 which is in circuit with the grid 80. Consequently the beam current through the tube 60 is caused to vary in accordance with changes in the average output level at the electrode 46. Thus. an increase in such output level acts to increase the positive bias of the electrode 80 and thereby to increase the beam current through the tube 80.

For regulating action to obtain, the beam current through the tube 60 should be initially adiusted so that the transmission line 50, it is in a slightly detuned condition when the signal level is at the desired value. Furthermore, the detuning should be in such a sense that an increase in beam current will tend still further to. increase it while a decrease in beam current will tend to improve the tuning. Under these conditions, a rise'in the output level at electrode 46 will act through the monitoring circuit to decrease the modulating eflect of the electrode 40. The resultwill be that the output level will be brought back toward the desired value. Conversely, a decrease in output level will increase the modulating ac,- tion of the-electrode ill and thereby correspondingly raise the output level.

The monitoring system described in the foregoing is eflective to correct the variations in average output due to any cause. However, it is especially advantageous in connection with an aniplifying device which employs modulation intensifying means such as is represented by the combination of the electrode 40 and the resonant lines 42, 43 and 50, 5|. The tuning of such an arrangement is extremely sharp so that its response to various frequencies is by no means uniform. Therefore, when the beam traversing the tube III is modulated by a signal which comprises both carrier and side band frequencies, a. relatively high degree of side band distortion may occur resulting in variations in outputlevel at the electrode 4. This efle'ct may be minimized,

'however, by the use of the monitoring arrangement which'I have described. Sincethe condition of resonance of the line 50, 5| is automatically adjusted in response to variations in output level which result from a shift in side-band fr q ency, the response of the system as a whole is rendered substantially independent of frequency.

While I have described my invention in connection with'a particular embodiment thereof it will be understood that numerous modifications may be made by those skilled in the art without departing from the invention. I, therefore, aim in the appended claims to cover all such equivalent variations as fall within the true spirit and scope of the ioreaoins disclosure.

What I claim as new and desire to secure by Letters Patentof the United States is:

1. In combination, means for producing an electron beam, means for modulating the beam at high frequency, resonant circuit means coupled to the modulated beam independently of the lation in accordance with the condition of'resonance of the circuit means, and means for varying the condition of resonance of the said circuitmeans in response to the occurrence of undesired changes in a characteristic of the modulated beam, thereby automatically to suppress or oii'set such changes.

2. In combination, means including a discharge device of the cathode ray type for producing an electron beam, means for modulating the beam with an input signal, resonant circuit means coupled to the beam and'adapted to be excited to oscillation thereby, said circuit means being effective when in oscillation to produce secondary modulation of the beam in accordance with the condition of resonance of the circuit means, out- .said modulating means and adapted to react with the beam so as variably to aifect its modumodulated beam, and means associated with the said output means for regulating the condition of resonance of the circuit means in response to variations in the output level, thereby to minimize such variations;

3. In combination, means including a discharge device of the cathode ray type for producing an electron beam, input means for modulating the beam with a high frequency signal, an electrode structure coupled to the modulated beam at a point along the beam path, the dimensions of the electrode structure being so correlated to the electron transit time in the beam as to assure eifective mutual reaction with the beam, resonant circuit means connected to the said electrode structure and adapted to be excited to oscillation by the action of the modulated beam thereon, the said circuit means being also effective to cause a secondary modulation of the beam which follows in pattern the initial modulation produced by the signal input means but which varies in magnitude in accordance with the condition of resonance of the circuit means, means for deriving output power from the doubly modulated beam and a monitoring system associated with the said output means for regulating the condi-' tion of resonance of the said circuit means in response to variations in the output level, thereby to prevent such variations from reaching objectionable proportions. v 4. In combination, means including -a discharge device of the cathode ray type for producing an electron beam, (means for modulating the beam with an input signal, resonant circuit means coupled to the beam and adapted to be excited to operation thereby, said circuit means being effective when in oscillation to produce secondary modulation oi the beam in accordance with the condition of resonance of the circuit means,-output means for deriving power from the doubly modulated beam at a level determined by the magnitude of the modulation, means for producing a second electron beam coupled to the said circuit means and effective to vary the condition of resonance thereof in accordance with current variations in such second beam, and means associated with said output means for regulating the current in the said second beam in response to variations in the output level, thereby to prevent such variations from reaching objectionable proportions. Y v v 5. In combination, means including a discharge device of the cathode ray type for producing an electron beam, means for modulating the beam with an input signal, a resonant type transmission line coupled to the beam and adapted to be excited to oscillation thereby, said line being effective wherr'in oscillation to produce secondary modulation of the beam in accordance with the condition of resonance of the line, output means for deriving power from the doubly modulated beam at a level determined by the extent of the modulation, means for producing a second electron beam coupled to the said transmission line and effective to vary the condition of resonance thereof in accordance with current variations in said second beam, and means associated with the said output means for regulating the current in the said second beam in response to variations in the output level, thereby to prevent such varia- 20 tions from reaching objectionable proportions.

6. In combination, means including a discharge device of the cathode ray .type for producing an efiective when in oscillation to produce secondary modulation of the beam in accordance with the condition of resonance of the line, output means for deriving power fromfthe doubly modulated beam, means for producing a second electron beam also coupled to the said transmission line, the said second beam being effective variably to load the line in accordance with variations in the current in such beam, thereby to vary the condition of resonance of the line, and means associated with the said output means for regulating the current in the said second beam in response to variations in the output level and in such iashion as to offset or suppress such variations.

R D. McARTI-IUR. 

